Re: [PATCH net] net/handshake: Fix null-ptr-deref in handshake_complete()

[Wang Liang <wangliang74@huawei.com>]

在 2025/12/10 2:06, Simon Horman 写道:
> On Tue, Dec 09, 2025 at 07:58:52PM +0800, Wang Liang wrote:
>> A null pointer dereference in handshake_complete() was observed [1].
>>
>> When handshake_req_next() return NULL in handshake_nl_accept_doit(),
>> function handshake_complete() will be called unexpectedly which triggers
>> this crash. Fix it by goto out_status when req is NULL.
>>
>> [1]
>> Oops: general protection fault, probably for non-canonical address 0xdffffc0000000005: 0000 [#1] SMP KASAN PTI
>> RIP: 0010:handshake_complete+0x36/0x2b0 net/handshake/request.c:288
>> Call Trace:
>>   <TASK>
>>   handshake_nl_accept_doit+0x32d/0x7e0 net/handshake/netlink.c:129
>>   genl_family_rcv_msg_doit+0x204/0x300 net/netlink/genetlink.c:1115
>>   genl_family_rcv_msg+0x436/0x670 net/netlink/genetlink.c:1195
>>   genl_rcv_msg+0xcc/0x170 net/netlink/genetlink.c:1210
>>   netlink_rcv_skb+0x14c/0x430 net/netlink/af_netlink.c:2550
>>   genl_rcv+0x2d/0x40 net/netlink/genetlink.c:1219
>>   netlink_unicast_kernel net/netlink/af_netlink.c:1318 [inline]
>>   netlink_unicast+0x878/0xb20 net/netlink/af_netlink.c:1344
>>   netlink_sendmsg+0x897/0xd70 net/netlink/af_netlink.c:1894
>>   sock_sendmsg_nosec net/socket.c:727 [inline]
>>   __sock_sendmsg net/socket.c:742 [inline]
>>   ____sys_sendmsg+0xa39/0xbf0 net/socket.c:2592
>>   ___sys_sendmsg+0x121/0x1c0 net/socket.c:2646
>>   __sys_sendmsg+0x155/0x200 net/socket.c:2678
>>   do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
>>   do_syscall_64+0x5f/0x350 arch/x86/entry/syscall_64.c:94
>>   entry_SYSCALL_64_after_hwframe+0x76/0x7e
>>   </TASK>
>>
>> Fixes: fe67b063f687 ("net/handshake: convert handshake_nl_accept_doit() to FD_PREPARE()")
>> Signed-off-by: Wang Liang <wangliang74@huawei.com>
>> ---
>>   net/handshake/netlink.c | 35 ++++++++++++++++++-----------------
>>   1 file changed, 18 insertions(+), 17 deletions(-)
>>
>> diff --git a/net/handshake/netlink.c b/net/handshake/netlink.c
>> index 1d33a4675a48..cdaea8b8d004 100644
>> --- a/net/handshake/netlink.c
>> +++ b/net/handshake/netlink.c
>> @@ -106,25 +106,26 @@ int handshake_nl_accept_doit(struct sk_buff *skb, struct genl_info *info)
>>   
>>   	err = -EAGAIN;
>>   	req = handshake_req_next(hn, class);
>> -	if (req) {
>> -		sock = req->hr_sk->sk_socket;
>> -
>> -		FD_PREPARE(fdf, O_CLOEXEC, sock->file);
>> -		if (fdf.err) {
>> -			err = fdf.err;
>> -			goto out_complete;
>> -		}
>> -
>> -		get_file(sock->file); /* FD_PREPARE() consumes a reference. */
>> -		err = req->hr_proto->hp_accept(req, info, fd_prepare_fd(fdf));
>> -		if (err)
>> -			goto out_complete; /* Automatic cleanup handles fput */
>> -
>> -		trace_handshake_cmd_accept(net, req, req->hr_sk, fd_prepare_fd(fdf));
>> -		fd_publish(fdf);
>> -		return 0;
>> +	if (!req)
>> +		goto out_status;
>> +
>> +	sock = req->hr_sk->sk_socket;
>> +
>> +	FD_PREPARE(fdf, O_CLOEXEC, sock->file);
>> +	if (fdf.err) {
>> +		err = fdf.err;
>> +		goto out_complete;
>>   	}
>>   
>> +	get_file(sock->file); /* FD_PREPARE() consumes a reference. */
>> +	err = req->hr_proto->hp_accept(req, info, fd_prepare_fd(fdf));
>> +	if (err)
>> +		goto out_complete; /* Automatic cleanup handles fput */
>> +
>> +	trace_handshake_cmd_accept(net, req, req->hr_sk, fd_prepare_fd(fdf));
>> +	fd_publish(fdf);
>> +	return 0;
>> +
>>   out_complete:
>>   	handshake_complete(req, -EIO, NULL);
>>   out_status:
> Hi,
>
> Clang 21.1.7 W=1 builds are rather unhappy about this:
>
>    net/handshake/netlink.c:110:3: error: cannot jump from this goto statement to its label
>      110 |                 goto out_status;
>          |                 ^
>    net/handshake/netlink.c:114:13: note: jump bypasses initialization of variable with __attribute__((cleanup))
>      114 |         FD_PREPARE(fdf, O_CLOEXEC, sock->file);
>          |                    ^
>    net/handshake/netlink.c:104:3: error: cannot jump from this goto statement to its label
>      104 |                 goto out_status;
>          |                 ^
>    net/handshake/netlink.c:114:13: note: jump bypasses initialization of variable with __attribute__((cleanup))
>      114 |         FD_PREPARE(fdf, O_CLOEXEC, sock->file);
>          |                    ^
>    net/handshake/netlink.c:100:3: error: cannot jump from this goto statement to its label
>      100 |                 goto out_status;
>          |                 ^
>    net/handshake/netlink.c:114:13: note: jump bypasses initialization of variable with __attribute__((cleanup))
>      114 |         FD_PREPARE(fdf, O_CLOEXEC, sock->file);
>          |                    ^
>
> My undersatnding of the problem is as follows:
>
> FD_PREPARE uses __cleanup to call class_fd_prepare_destructor when
> resources when fdf goes out of scope.
>
> Prior to this patch this was when the if (req) block was existed.
> Either via return or a goto due to an error.
>
> Now it is when handshake_nl_accept_doit() itself is exited.
> Again via a return or a goto due to error.
>
> But, importantly, such a goto can now occur before fdf is initialised.
> Boom!


Thanks for your analysis, you are right!

How about adding a null check before calling handshake_complete()?

Like:

diff --git a/net/handshake/netlink.c b/net/handshake/netlink.c
index 1d33a4675a48..b989456fc4c5 100644
--- a/net/handshake/netlink.c
+++ b/net/handshake/netlink.c
@@ -126,7 +126,8 @@ int handshake_nl_accept_doit(struct sk_buff *skb, 
struct genl_info *info)
         }

  out_complete:
-       handshake_complete(req, -EIO, NULL);
+       if (req)
+               handshake_complete(req, -EIO, NULL);
  out_status:
         trace_handshake_cmd_accept_err(net, req, NULL, err);
         return err;

------
Best regards
Wang Liang